1. Field of the Invention
The present invention relates to the field of permanent magnets. More specifically, the present invention relates to the field of multipole or dipole permanent magnet (PM) structures for generating an intense magnetic field in a gap using a minimal volume of magnet material for the permanent magnet structure, wherein the magnetic field intensity in the gap varies according to a substantially linear gradient along the longitudinal axis of the gap.
2. Description of the Related Art
Introduction
The present invention relates to a configuration of a plurality of permanent magnets to produce a permanent magnet (PM) structure capable of generating in an aperture or gap formed by the permanent magnets a magnetic field having a high flux density that varies in a substantially linear manner along the longitudinal axis of the gap.
The performance of a permanent magnet depends on the magnet itself and the environment in which it operates. Advances in permanent magnetism have had a large impact on the number of applications for which permanent magnets may now be used or considered. Advances in such areas as magnet material (for example, rare earth magnet materials), magnet size, and magnet structure have combined to produce permanent magnets having internal magnetic fields with very high flux densities, for example, above 1.4 Tesla (14,000 Gauss). Indeed, today the properties exhibited by permanent magnets offer compelling reasons to use permanent magnets over electromagnets.
Electromagnets can produce quite large magnetic fields by driving electrical current through a coil of electrically conductive wire. However, the size and expense of such electromagnets, as well as power supply requirements and heat dissipation problems, make electromagnets unattractive for applications requiring an intense magnetic field in a physically small space.
Permanent magnets are used in applications that exploit the permanent magnet's unique capability to provide a force, or perform work of some kind without contact. In order for a permanent magnet to perform work, it must generate a magnetic field external to itself. Typically, the object upon which the permanent magnet operates is placed or passes through an aperture or air gap, or simply, gap, in the magnetic circuit formed by the permanent magnetic structure. The greater the strength of the magnetic field capable of being generated by the permanent magnet structure in the gap, the greater the permanent magnet's ability to perform work. To that end, research has focused on techniques to improve the efficiency of the magnetic circuit formed by the permanent magnet structure so as to maximize the strength of the magnetic field in the gap while minimizing the volume of magnet material required.
There are many prior art permanent magnet structures, from the ubiquitous C-shaped dipole permanent magnet to complex multipole permanent magnet structures designed for highly specific applications, for example, synchrotron radiation, or the operation of free electron lasers. Yet some applications, such as spectrometers based on exploiting the Zeeman effect, or the field of power generation known as magnetohydrodynamics, require magnetic field intensities unattainable within the design limitations imposed by such applications using prior art permanent magnet structures due substantially to leakage flux and fringing flux, as briefly described below.
Leakage and Fringing Flux
A brief overview of prior art permanent magnet structures and their limitations with respect to leakage flux and fringing flux is beneficial for understanding the present invention.
An efficient design of a permanent magnet should minimize the effects of leakage flux and fringing flux. Minimizing leakage flux and fringing flux can be accomplished by recognizing and accommodating in the design of the permanent magnet structure the following principles:
1. Magnetic lines of force (flux lines) follow the path of least reluctance (the reciprocal of permeance). Thus, for example, flux lines will generally flow more easily through ferromagnetic materials than air because, as is well known, ferromagnetic materials have a higher permeance than air. PA0 2. Flux lines flowing in the same direction repel one another. Thus, magnetic lines of force tend to diverge as they move away from a magnetic pole rather than converge or remain parallel. PA0 3. Flux lines always form closed loops and cannot, therefore, intersect. PA0 4. Flux lines represent a tension along their length which tends to make them as short as possible. Thus, given that flux lines also form closed loops, they always form curved lines from the nearest north pole to the nearest south pole in a path that forms a complete closed loop. It should be noted that flux lines do not necessarily pass from the north pole to the south pole of the same magnet, but may go from the north pole of a first magnet to the south pole of a second magnet that is either physically closer to the north pole of the first magnet or there is a lower reluctance path from the north pole of the first magnet to the south pole of the second magnet than the path from the north pole to the south pole of the first magnet. PA0 5. In a magnetic circuit, any two points of equal distance from a neutral axis essentially function as poles, wherein flux lines exist between them.
Keeping the above principles in mind, and with reference to FIG. 1, a permanent magnet structure 100 is illustrated in which permeable pole pieces 102 and 103 (which may be made of, for example, mild steel), permanent magnet 101, and air gap 104 form a magnetic circuit. Fringing flux near air gap 104 passes around the air gap, as illustrated by flux lines 105, primarily because of principles (1) and (2) above, rather than directly through the air gap, as illustrated by flux lines 107. Leakage flux, as illustrated by flux lines 106, flows between pole pieces 102 and 103 and across the back of the magnetic circuit from the north pole to the south pole of magnet 101, primarily because of principles (1), (4) and (5) discussed above.
As illustrated in FIG. 1, the total flux directly through the air gap is less than the total flux in the magnetic circuit formed by permanent magnet structure 100 because of the effects of fringing flux and leakage flux. The magnetic field intensity (H) present in air gap 104 is directly related to the number of lines of flux, i.e., the flux density (B), within air gap 104, based on the equation: EQU H=.mu.B
where .mu. is the permeability of, in this case, air (a constant). Thus, the greater the number of lines of flux passing directly through the air gap, i.e., the greater the flux density (B) in the air gap, the greater the magnetic field intensity (H) in the air gap.
Techniques that minimize fringing flux and leakage flux improve the efficiency of the magnetic circuit formed by a permanent magnet structure by increasing the magnetic field intensity (H) in the air gap where it is desired in order to perform work. FIGS. 2(a), (b), (c), and (d) illustrate four prior art methods of minimizing leakage flux. FIG. 2(a) illustrates optimizing the shape of the permanent magnet. Magnet 201 is optimized to minimize leakage flux occurring in magnet 200. FIG. 2(b) illustrates optimizing the location of permanent magnets within a magnetic circuit. While magnet 211 is an improvement over magnet 210, magnet 212 is the best configuration for reducing leakage flux. FIG. 2(c) demonstrates using blocking poles or blocking magnets to reduce leakage flux in the area in which the blocking pole is placed. The use of blocking poles is based on the principle that flux lines from like poles repel each other. Thus, leakage that may occur across the inside area of horseshoe magnet 220 is minimized by inserting a bar magnet 221 (having, importantly, a magnetic orientation opposite to magnet 220, with like poles abutting, thereby providing a counter magnetomotive force) in the inside area formed by magnet 220. The same principle applies to the placement of blocking magnets 223 and 224 about bar magnet 222--the presence of properly oriented permanent magnets at the appropriate position in the magnetic circuit reduce leakage flux and, as a result, increase flux density where desired, e.g., in an air gap. Finally, FIG. 2(d) illustrates optimizing the magnetic field orientation, i.e., aligning the magnetic lines of force with respect to the physical dimensions of the permanent magnet 231 to achieve a more efficient magnetic circuit than in the case of magnet 230.
Notwithstanding the above methods for reducing leakage flux and fringing flux, the flux density of the external magnetic field in the air gap is still limited by the leakage of flux to some fraction of the intrinsic flux density of the magnet material used. To increase the flux density in the gap, it is well known to those of skill in the relevant art to collect and concentrate the available flux in the circuit by using permeable pole pieces, which may be tapered in the direction of the air gap. Generally, the permeance of an air gap is directly proportional to the area of the gap and inversely proportional to the length of the gap. Increasing the air gap area or, more preferably, reducing the length of the gap will increase the permeance of the gap. The tapering of the pole pieces, in contrast, increases the length of the path along the edge of the gap, where the fringing flux passes.
Tapering the pole pieces decreases the permeance at the edge of the air gap and, as a result, decreases the fringing flux. However, this increases the magnetic potential at the pole piece edges, and much of the available flux is lost to intramagnet leakage, as illustrated in FIG. 3 at 306. In FIG. 3, a prior art H-shaped dipole permanent magnet structure 300 is comprised of a yoke 301 made of, for example, a permeable steel alloy, and two permanent magnets 302 and 303. To each of the permanent magnets is coupled a tapered pole piece 304 and 305, respectively, made of high permeability alloy. Air gap 308, through which flux lines 307 directly pass, completes the magnetic circuit. Because the pole pieces are made of high permeability alloy, and due to the reluctance of the air gap, the flux density along the beveled sides of the pole pieces increases. For example, the increase in flux density along a beveled side of pole piece 304 increases the magnetic potential across the magnet 302 and causes flux to leak back over the surface of magnet 302, as illustrated by flux lines 306. Thus, it can be seen that tapered pole pieces may not provide as much of an increase in gap flux density as desired due to intramagnet leakage.
With reference to FIG. 4, a prior art H-type dipole permanent magnet structure 400 improves upon the structure of FIG. 3 by placing blocking magnets (403, 404, 405 and 406) between pole pieces (407, 408, 409 and 410) and the yoke 401. In so doing, flux from the blocking magnets prevents leakage from the pole pieces back to the permanent magnets (402 and 403), or from the pole pieces to the yoke, thereby contributing to the total flux available (flux lines 412) at the gap 411. Leakage due to fringing flux is not entirely prevented due to the open areas to the side of air gap 411 into which the magnetic field in the air gap expands, reducing flux density in the air gap.
Although the flux density (B) of the external magnetic field in the air gap of the permanent magnet structure in FIGS. 3 and 4 is greater than the flux density in the air gap of the structures illustrated in FIGS. 2(a), (b), (c), and (d), B is still limited by the leakage of flux to some fraction of the intrinsic flux density of the magnet material used. The prior art permanent magnet structure of FIG. 5(a) further increases the flux density in an air gap through the superposition of the magnetic fields of each of the trapezoidal-shaped permanent magnet segments.
With reference to FIG. 5(a), a cross sectional view of a prior art dipole permanent magnet structure is illustrated. A plurality of trapezoidal shaped permanent magnet segments 502 are arranged about a longitudinal axis within a cylindrical yoke 501, forming a cylindrical air gap 503 along the center of the axis. The orientation of the magnetic field 504 of each segment 502 is positioned with respect to the orientation of the magnetic field of an adjacent segment to complete a magnetic circuit through the segments, thereby forming a uniform dipole magnetic field 505 in air gap 503 perpendicular to the longitudinal axis. FIG. 5(b) illustrates the effect of superpositioning the magnetic field 504 of each segment 502.
The permanent magnet structure 500 illustrated with reference to FIG. 5(a) forms a ring geometry with concentric inside and outside diameters in which the magnetization vector continuously rotates from pole to pole. In practice this geometry is approximated by an assembly of trapezoids 502 cut from generally rectangular or square blocks of magnet material. The blocks, before being cut, have a magnetic orientation straight through the block as induced during manufacturing or during the magnetization process for isotropic materials. With planning, the resulting trapezoids will have a magnetic orientation such that the magnetic vector components of each trapezoid will, by superposition, add to create the desired gap flux density 505 (FIG. 5(b)) in the round aperture or cylindrical air gap 503.
The prior art permanent magnet structure in FIG. 5(a) provides a very uniform magnetic field in the central two-thirds (2/3) of the interior diameter of air gap 503. However, a gap flux density greater than the residual flux density (B.sub.r) of the magnet segments 502 may cause the inside corners of the segments to be exposed to a magnetic field whose intensity is greater than the intrinsic coercivity of the magnet material used in the segments. Such exposure can reverse the direction of magnetization in the corners of the segments, limiting the maximum flux density of the air gap. Furthermore, unlike the prior permanent magnet structures shown in FIGS. 3 and 4, ferrous material cannot be used in the permanent magnet structure of FIG. 5(a). Coupling permeable pole pieces to segments 502 in gap 503 would cause flux to be shunted through the pole pieces situated around the air gap rather than through the gap, lowering the flux density of the gap rather than increasing it. Thus, the maximum flux density of the air gap is proportional to the residual flux density of the magnet material used in the segments times the natural log of R.sub.o /R.sub.i, and factors for the number of segments used and the proximate length of the structure, where R.sub.o is the outside radius of the structure and R.sub.i is the inside radius of the structure.
Yet another limitation of the prior art permanent magnet structure shown in FIG. 5(a) is that the geometry is not well suited to applications requiring a rectangular aperture. When a square or rectangular gap is required for a given application involving a permanent magnet structure, the inner diameter of the structure of FIG. 5(a) must circumscribe the square or rectangular aperture. To generate a magnetic field in the air gap having a very high flux density of, e.g., 2 Tesla, the magnet structure of FIG. 5(a) needs approximately 35% more magnet material than a corresponding structure such as illustrated in FIGS. 6 and 7.
It is evident from the above discussion that an external magnetic field in a rectangular or square gap having a very high flux density or a flux density greater than the residual flux density (B.sub.r) of the magnet material employed generally cannot be produced economically with the prior art dipole permanent magnet structures discussed thus far. The dipole permanent magnet structure illustrated in FIGS. 6 and 7, as described in U.S. Pat. No. 5,635,889, assigned to the assignee of the present invention, and incorporated herein by reference, can achieve high magnetic field intensities, for example, flux densities above 2 Tesla (20,000 Gauss).
The dipole permanent structure illustrated in FIGS. 6 and 7 is a dipole magnet structure capable of generating an external magnetic field in an air gap whose flux density is greater than the residual flux density of the magnet material employed in the dipole magnetic structure. According to the prior art permanent magnet structure illustrated in FIGS. 6 and 7, the flux density of the external magnetic field in the air gap is limited by the saturation flux density of the permeable material used in the pole pieces rather that the residual flux density of the magnet material used in the permanent magnets. However, the prior art permanent magnet illustrated in FIGS. 6 and 7 is limited in that the air gap is suitable only for certain applications requiring a rectangular or square aperture.
Additionally, the prior art dipole permanent magnet structure illustrated in FIGS. 6 and 7 is limited in that it can only provide a very limited range of flux densities in the rectangular gap. Some applications need a permanent magnet structure capable of providing a range of flux densities, from relatively low flux densities to very high flux densities. For example, in bolometers utilized in plasma diagnostics, or cryogenically cooled detector systems for detecting infrared and millimeter-wave frequencies, and other types of wideband instrumentation, it is beneficial to offer superior sensitivity, in terms of the operating frequency range. The frequency range is set, in part, by positioning a detector crystal in a specific magnetic field density ranging from 0.5 to 2.0 Tesla. In the prior art, dipole permanent magnet structures provide a very limited range of flux densities. Thus, a different dipole permanent magnet structure must be utilized for each desired frequency. What is needed is a single dipole permanent magnet structure capable of providing a magnetic field in a gap having a range of magnetic flux densities.